My name is Steven, I am a undergraduate student doing a computational project using starccm+. My model is a blow-down supersonic wind tunnel which has an inlet and an outlet. The total length of this tunnel is 5.5 m, including an inlet, a settling chamber, a convergent-divergent nozzle, an 1 m long working section, and finally a 2.5 m long diffuser.

Firstly I ignored the settling chamber and modeled this wind tunnel from the nozzle to diffuser in order to fasten the computation. So I defined the beginning plane of nozzle as the inlet and the exit of diffuser as the outlet. And the inlet is 'Stagnation Inlet', whose supersonic static pressure is 102300 pa, total pressure is 184140 Pa. The outlet is 'pressure outlet', which is 102300 Pa. Then after running the computation, I can get the supersonic flow in the working section and a normal shock wave at the end of working section, which is also the inlet of diffuser.

Now I tried to add the settling chamber in front of the nozzle and move the inlet to the beginning of settling chamber. Then I run the computation with the identical boundary conditions and the parameters, however I could only obtain the subsonic flow over the entire wind tunnel. I have tried to increase the inlet static pressure and total pressure or decrease the outlet pressure, but I still could only obtain the supersonic flow at the first hundreds of iterations, then when the residual started to converge, the flow drops to entirely subsonic again. Could you please tell me how to obtain supersonic flow inside this extended wind tunnel geometry? Thank you very much!!!

Best regards,

Steven

PS: Firstly I thought it is a 'wind tunnel starting' problem, so I increased the inlet total pressure to as high as 300000 Pa, but still obtained the entire subsonic flow in the wind tunnel, so I believe it should be a numerical problem of Starccm+, because I further tried different 3D mesh types (e.g. polyhedral, trimmer and triangle meshes) under different total pressure/ exit pressure combinations, the results are very different from case to case, some even diverged.

Were you able to solve your problem? I also need some help in supersonic wind tunnel CFD. I am designing a Mach 2 wind tunnel, and I am not getting the right results in my CFD. The shock wave is always at the first throat even though I increase the total pressure at inlet. Please suggest the possible solution. Thank you.